1. Technical Field
The present invention relates generally to vehicle air conditioning systems, and particularly to a refrigerating cycle apparatus in which a high temperature/high pressure gas refrigerant discharged from a refrigerant compressor is guided through a pressure reducer and then an internal heat exchanger to heat air flowing through an air conditioner duct.
2. Related Art
A conventional vehicle air conditioning system includes a hot-water warming unit in which engine cooling water is guided to a hot-water heater located in an air conditioning duct to heat air flowing through the duct. However, such a hot water warming unit is inefficient during engine startup when both outside air and cooling water temperatures are low.
Japanese Patent Application Laid-Open No. Hei. 5-223357 describes a vehicle air conditioning apparatus having an auxiliary warming unit adapted to assist the main warming unit by guiding a high temperature/high pressure gas refrigerant discharged from an engine-driven refrigerant compressor to an internal heat exchanger via a pressure reducing unit, so that the heat exchanger heats the air flowing through an air conditioning duct.
When cooling water temperature is higher than a predetermined temperature and the warming power of the main warming unit is sufficiently high, the vehicle interior is sufficiently heated, and the refrigerant compressor is turned OFF to stop the hot gas heater cycle. Further, when the cooling water temperature is lower than the predetermined temperature and the warming power of the main warming unit is therefore insufficient, the refrigerant compressor is turned ON to start the hot gas heater cycle.
The hot gas heater cycle of the above system is adapted such that heat generated by compression work of the refrigerant compressor is radiated by a vehicle internal heat exchanger. The heat is radiated in a manner different from a typical heat pump cycle in which the vehicle internal heat exchanger functions as a refrigerant condenser, and in which a vehicle external heat exchanger functions as a refrigerant evaporator. Therefore, the cycle can operate even when the outside air temperature is very low, such as in the vicinity of -40.degree. C.
However, in the above heater cycle, when the outside temperature is around -20.degree. C., negative pressure forms due to characteristics of refrigerant HFC-134a before the refrigerant compressor is started. Further, due to characteristics of the hot gas heater cycle, when the suction portion of the refrigerant compressor is at a negative pressure, suction pressure (PS) of a compressor suction portion just after the compressor starts typically decreases initially, as shown in FIG. 7, before gradually increasing. As a conventional engine-driven refrigerant compressor typically has a weak shaft seal, if compressor suction pressure decreases below a predetermined value (for example -0.5 kg/cm.sup.2 G), air tends to penetrate into the compressor from the seal portion. Such air penetration is undesirable, as it tends to cause corrosion and high pressure to abnormally increase.